At first I thought it was too closed circuit, not enough options for routes.
But in gameplay, its fast and has nice little open areas around the bases.

This is almost a "catacomb" more than it is a sewer.
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A nicely done map, however there could be a few things that occur with a lot of people. The map becomes rather small when moving anywhere out of a base. I am sure this is what SourPatch meant by "catacomb".
Also, once filled, which the map inevitably will be, it becomes a shotgunners paradise. One way this issue can be solved is in putting more width into hallways.

Overall i can see that you have made some good texture choices throughout the map, and the lighting has not been overdone thus giving the map atmosphere, however, i am still unsure about why there are so many boxes in a "sewer".

I really like the ambiance of this map, the textures suit the architecture! It may be a bit small (tight in places) but I bet it will be fun to play on.